1. Field of the Art
This invention relates to a process for producing propylene block copolymers under the conditions where substantially no solvent is used in an improved yield, which polymers have high rigidity and high impact strength with good powder flowability.
2. Related Art
Crystalline polypropylene, while having excellent characteristics of rigidity and heat resistance, had on the other hand the problem of low impact strength, particularly low impact strength at a low temperature.
As the method for improving this point, there have been already known a method of forming a block copolymer by polymerizing stepwise propylene and ethylene or another olefin (Japanese Patent Publications Nos. 11230/1968, 16668/1969, 20621/1969, 24593/1974, 30264/1974, Japanese Laid-open Patent Publications Nos. 25781/1973, 115296/1975, 35789/1978 and 110072/1979).
However, when propylene and ethylene are polymerized in two stages or multiple stages, although impact resistance may be improved, there ensues on the other hand the problem that polymers of low crystallinity are formed in a large amount, because the product contains copolymer portions.
Also, for improvement of impact strength of a block copolymer, it has been generally practiced to increase an amount of a rubbery copolymer formed in the block copolymerization. However, formation of by-products may be increased, and the polymer particles tend to have increased tackiness with the increase of rubbery copolymers, whereby sticking between polymer particles or sticking of the polymer particles onto an inner wall of a reactor may occur, whereby it becomes frequently difficult to perform stable running of a polymer production reactor for a long term.
More particularly, poor fluidity of the polymer particles due to sticking of polymer particles will present a serious problem to operation of a process wherein no solvent is used, such as, e.g. gas-phase polymerization.
Accordingly, it has been desired to develop technology by which sticking of polymer particles is prevented when the content of a rubbery polymer is increased thereby to increase process stability.